It has been shown that rapid local response to medical emergencies is typically the determining factor as to survivability and quality of life after a critical cardiac or respiratory event. State-of-the-art hospital ecg monitors, apnea monitors, and oximeters and home-use personal emergency response systems (PERS) have not yet succeeded in providing a cost-effective fault-tolerant safety-net for the majority of patients who are at risk for sudden death.
The success of medical monitoring in the OR/ICU/CCU and other well-staffed hospital units is well-documented. It is in the under-staffed medical/surgical hospital units, the emergency room, the alternate site (nursing homes, surgi-centers, ambulatory care facilities) or the home environment (including retirement communities), that success is still elusive. Observers have suggested that while contemporary monitors do well in minimizing false-negatives, unattended patients and their monitors produce a challenging number of false-positives.
The major cause of false-positives in most monitors (other than incorrect sensitivity settings) is artifact. Artifact is typically generated by: patient activity (i.e. muscle generated noises, lead movement, rubbing of skin and clothing over leads or electrodes); or by electromagnetic interference (EMI).
Single parameter or single sensor systems do not typically have the capability to cross-check multiple recording sites for optimal artifact rejection. An array of sensors and/or a multiparameter approach could address some of these problems. In typical impedance pneumography, two parameters are derived from one set of trans-thoracic electrodes: heartrate and respiration effort. Since the two parameters derived are dependent upon a signal which is similar in nature but of lesser magnitude than other bodily activity, the desired signal is overwhelmed by such artifact, resulting in a false-positive alert, or a, on occasion, false-negative condition.
A number of companies have devised strategies to deal with this problem. For example, Nellcor utilizes a separate ecg channel to confirm that their oximeter sensor's pulse/O.sub.2 data is not invalidated by artifact. However, a single channel ecg itself is seriously prone to movement artifact and thus even improved oximeters are best utilized in minimal-activity environments. Continuous noise-tolerant ambulatory monitoring still challenges that method.
In most prior monitors the link between the sensor and signal processor is vulnerable to movement artifact. A number of successful monitoring schemes (i.e., an electrode , manufactured by Heart Rate, Inc. of Costa Mesa, Cal., under license from NASA) have provided for preliminary signal processing at the sensor level. The use of a hardwire from a combined sensor/signal processor to the "decision making" portion of the system however still presents a problem. This and the large size of the unit limits the patient's mobility and comfort.
Radiotelemetry has been proposed, for example in U.S. Pat. No. 4,827,943, issued May 9, 1989, and U.S. Pat. No. 4,784,162, issued Nov. 15, 1988, naming several of the inventors common to the subject application. U.S. Pat. No. 4,827,943 also discusses "on-body decision making." Although using radiotelemetry with "off-body decision making" is likely to eliminate the hard-wire artifact and reduce the discomfort problem, it introduces another problem, mainly that of the vulnerability of radiotelemetry.
Radiotelemetry, although extremely useful, by its nature exhibits several weaknesses. It is prone to "drop-out" caused by the patient's on-body transmitting antenna orientation shifting in relation to the base-station receiver antenna orientation. This can cause a "null" condition or failure to maintain equal signal strengths. Thus a patient can orient his/her body in such a way as to weaken the radio-link causing false-alerts or even totally losing monitoring protection. Other failure in the radio link can be caused by going "out-of-range" or being near large unshielded electrical appliances or metal walls. Continuous telemetry devices also consume large amounts of power and are typically too large to wear comfortably for long-term monitoring.
Many cardiologists and internists express frustration with the technical limitations of existing Holter monitors. Electrode interface integrity during long-term ambulatory monitoring is often poor and results in unacceptable studies. Further, it is felt that many cardiac abnormalities go undetected because they are not likely to occur during the typical 24-hour period of monitoring. Finally, Holter monitors are intended for diagnostic use and do not in themselves provide "real-time" protection for the patient.
It is therefore desirable to have a cardiorespiratory alert system for monitoring in hospitals (med-surg units); alternate sites (nursing homes, surgi-centers, ambi-centers); and homes (including retirement communities) which is cost-effective and comfortable, and which provides reliable protection to the patient and generates a minimum of false positives. Additionally, it is desirable to have an artifact and fault-tolerant system for operating in high noise environments. It is also desirable to have a system which is catastrophic-failure resistant.